Telemetry may be defined as the science of sensing information at some remote point-of-measurement location and transmitting data representing the sensed information to a convenient receiving station to be analyzed. In a common type of telemetric measuring system, appropriate sensors are used to acquire data at the point of measurement, and the data is converted to a radio frequency signal, which is transmitted to a receiver at the user station.
One application of telemetry is the measurement of physical conditions, such as temperature, stress, and pressures, in mechanical engines. This application presents special problems. The engine motion tends to result in hostile conditions, such as high temperature and shock. Also, many engines have limited space for installing the measuring device.
As with other measuring devices, the transducers used for telemetric measuring devices can be either "passive" in the sense that they derive energy from the measurand, or "active" in the sense that they derive energy from a power source. Most active transducers rely on power from a battery or other conventional power source external to the measurand.
A self-powered active telemetric measuring device, that measures physical conditions associated with machinery, is described in U.S. Pat. No. 3,596,262. It has a power generator that uses the motion of the machinery to sling a magnet back and forth, thereby generating alternating electrical current. Power from this current is used in a transmitter circuit, having an oscillator whose frequency is a function of the parameter being measured. A capacitor, used as a tuning capacitor in the transmitter circuit, changes its capacitance according to temperature.
A design consideration in telemetric measuring devices, such as that described in U.S. Pat. No. 3,596,262 and others, is the variety of measured data that may be acquired. It would be advantageous to provide a measuring device that acquires temperature data from more than one sensor point or acquires two different types of data, such as temperature and pressure. Thus, there is a need for a telemetric measuring device having a number of input channels, each receiving different sensed data. Ideally, the device must operate at high temperatures, withstand high acceleration forces, and be durable and easily installed.